Modes of electron tunnelling through semiconductor devices, preferably DC biased quantum well structures, are well known. Appl. Phys. Lett. 22, 562 (1973). Normally, the main contribution to the tunnelling current is assumed to come from elastic, nonradiative transitions, but the presence of a potential difference and the charge of the carriers undergoing tunnelling make radiative, inelastic tunnelling transitions unavoidable. See FIG. 2. Relative suppression of radiative processes by one power of the fine structure constant e.sup.2 (electron charge squared), as well as enhancements associated with stimulated emission, is generally expected.
Both radiative and nonradiative tunnelling in such semiconductor devices can involve exponentially small tunnelling factors, but at least for emission of soft photons, no substantial exponential relative suppression of radiative tunnelling transitions is expected. Thus, comparisons between radiative and nonradiative processes reduce to a matter of phase space and matrix element considerations particular to planar quantum well geometry. This involves coupling of carriers to the electromagnetic field taken to the nonzero within the portion of the quantum well structure which is sandwiched between heavily doped planar contact layers and within the skin depth in the heavily doped layers of the quantum well structure.
Interband transitions in semiconductor devices occur with wide energy gap materials, such as gallium arsenide, GaAs. See FIG. 1. Where a narrow gap is desired, materials such as Hg.sub.x Cd.sub.1-x Te are employed. These, however, have proved to be troublesome in practice. A potentially useful feature of the spectrum of electromagnetic radiation which is made available from intraband tunnelling transitions in semiconductor devices is that there is no long wavelength cutoff.
It should thus be possible to fabricate far infrared or microwave radiation sources with fewer materials problems than is possible with devices employing interband transitions in narrow energy gap materials. The spontaneous emission spectrum should be smooth, whereas the stimulated emission spectrum will be peaked.
When an electron tunnels through a quantum well structure, or a finite superlattice, the electron can emit a photon, or absorb a photon, within the quantum well structure and emerge with a lower, or higher, energy. One such schematic model is depicted in FIG. 1. The electromagnetic vector potential is considered with the polarization vector in a direction perpendicular to the superlattice for electromagnetic waves propagating in the plane of the quantum well, or superlattice layers. Propagation in the plane of the quantum well is believed to occur because of the boundary conditions which are imposed by planar doped, conducting layers on each side of the quantum well structure.
It is an object of the present invention to provide semiconductor devices adapted to operate in the intraband tunnelling current transition mode in which there is no inherent long wavelength radiation emission cut-off. It is another object of the invention to provide for far infrared and microwave radiation sources having fewer material of fabrication problems than has been possible with present semiconductor devices utilizing narrow energy gap materials. It is a still further object of the invention to produce a semiconductor device wherein the tunnelling current establishes a photon creation rate which is in excess of the ohmic damping loss rate. It is a further object of the invention to provide a semiconductor device structure which shows an increased coupling of the tunnelling current to the electromagnetic field in the region between the impurity-doped layers which define the boundaries of an intraband transition device. It is yet another objective to provide a semiconductor device, preferably a quantum well structure, capable of operating in a laser mode for stimulated photon emission.